The present disclosure relates to a non-contact coordinate measuring machine (CMM) incorporating an electro-optical sensor using a new structured light code. Some past CMM's have made use of mechanical stylus probes which contact the surface being measured at a single point. Such machines can measure a few selected coordinates on precision-manufactured parts with an accuracy of the order of 1-5 microns (0.00004 to 0.0002 inches). However, there is growing interest in the possibility of replacing the mechanical stylus with a non-contact optical sensor of comparable or better accuracy, faster measurement, and a denser spacing of measurement points.
Non-contact three dimensional (3D) optical sensors using a single scanned projected laser line are now commercially available. One or two digital cameras view the reflected light from the object's surface at a different angle from the laser light direction, enabling the distance to points on the surface to be measured by triangulation. Although this arrangement increases the area scan rate beyond that possible with a stylus probe, the reported accuracy of currently available sensors of this type does not yet appear to match the accuracy of traditional stylus-type CMM's.
Sequential projection of structured light patterns covering more area than a single laser line on the surface being measured has been considered as a potential means to increase area scan rate and accuracy. One type of structured light that has recently received discussion and experimentation is based upon an optical implementation of the binary Gray code. See for example “Accurate Point Cloud Generation for 3D Machine Vision Applications using DLP® Technology and Industrial Camera”, Texas Instruments Application Report TIDA-00254 Test Results, June, 2014.
Projected Gray code patterns are two-dimensional clear and opaque grating patterns, alternately called bar patterns, and sometimes stripe patterns. There are several unique bar patterns in the Gray code sequence, each with a different periodicity. The coded dimension is perpendicular to the long side of the bars while the length of the bars may be changed as desired to change the area coverage. There is a potential scan rate advantage with Gray code structured light as compared to a single projected laser line system because of its area coverage.
More background concerning the Gray code is provided by the reference “Origins of the Binary Code”, F. G. Heath, Scientific American, August 1972. In that reference, the author states that what is now called the Gray code was first used in France in the mid to late 19'th century by Emile Baudot to eliminate ambiguities in coding and print-out of telegraphic signals. Baudot's name for the code is reported to be “cyclic-permuted code”.
On Mar. 17, 1953, Frank Gray of the Bell Telephone Laboratory was granted U.S. Pat. No. 2,632,058, in which he claimed a “reflected binary code”, now generally called the “Gray code”. It should be noted that this use of the word “reflected” refers to an even symmetry in the code pattern geometry rather than any physical ability to reflect light. Although Gray did not describe his code as cyclic or periodic, it can be seen in FIG. 2 of U.S. Pat. No. 2,632,058 that it is periodic and therefore also cyclic. It is “reflected” for all patterns except the most significant one, which has a single knife edge transition from black to white and was designated by Gray as a sign bit.
It can also be seen from FIG. 2 in U.S. Pat. No. 2,632,058 that the shortest period of a cyclic Gray code used for any purpose is equal to the width of four digital resolution elements. When that code is used to create structured light, an ideal spatial waveform of the minimum period pattern is a square wave having exactly one-half period with full illumination and exactly one half period with zero illumination.
Although many CMM's have been developed and are useful, there is an interest in new CMM technologies with reduced measurement error.